The present invention relates to a sector mark signal control circuit for a disc apparatus, and more particularly to the generation of a sector mark signal in an optical disc apparatus where coded information is recorded on an optical disc in such a manner that each of sectors formed on the optical disc is used as a recording unit.
Unlike the case where analog information (such as image data and video information) is recorded in an optical disc, it is desirable to record coded information (that is, digital information) on an optical disc so that data having a predetermined length can be used as an access unit. Accordingly, each track on the optical disc is divided into a number of equal sectors, and each of the sectors is assigned an address, to be used as an access unit in both a recording operation and a reproducing operation. The technique of this kind is disclosed, for example, in U.S. Pat. No. 4,363,116, and in the Nov. 21, 1983 issue of the "NIKKEI ELECTRONICS" on page 195 (pp. 189-213).
In the above-mentioned recording/reproducing method, however, in order to surely perform a recording/reproducing operation for an addressed or selected sector, it is required to correctly detect the starting position of the addressed sector. Accordingly, a mark formed of a special pattern (namely, a sector mark) is previously recorded on the optical disc at the starting position of each sector, and a sector mark signal (hereinafter referred to as "SM signal") is generated each time the sector mark is detected, to control a data recording operation or data read-out operation.
In some cases, however, owing to a strain on the optical disc, an erroneous tracking operation, the deterioration of a recorded pattern, and others, the sector mark cannot be detected though the sector mark is scanned by read means such as an optical head.